Receive Loop Antenna for 500kHz

I have good receive results on 500kHz, using my 40m Doublet with feeder strapped and no ATU I have copied the USA WSPR stations when only one other UK station was Spotting them also.
My TX antenna works well for RX too, yet with much discussion and some good results being reported using Indoor Loops I decided to try one.
I find one major problem when used indoors, despite the fact that the Loop can be oriented to reduce the pick up of interference from my Computer Monitor in the same room, the QRM from the Neighbour's Panasonic Plasma TV is severe and makes the band completely un-usable.

I hated the whole concept of Plasma TV's before one moved nearby. Despite the fact that I have many Panasonic Goods, I now despise that company for producing such poor equipment. In most cases of Plasma QRM it is Panasonic to blame.

In this wonderful enlightened age, the 42 inch Plasma TV is used in conjunction with a Dummy in the mouth of pre school children to rear and educate them with ranting American Cartoons on the Cartoon Channel. When the bleary eyed child decides he must sleep, the family continue to watch until the early hours. Therefore the QRM can seem to be 24/7 at times.

Other than that the Loop performance is impressive. I designed mine with 8 turns of 48 strand Litz wire spaced 10mm on a wooden frame.


It is tuned with a salvaged 3 Gang 500pF tuning capacitor. Coupling is by means of a toroid, just visible on the bottom mid turn in the picture below.


On a short test before the Loop was optimised, when the Child Rearer was off, I Spotted PA0A on WSPR in the daytime.

Transceivers RX Performance on the 600m Band

I have been confused since my very first listening sessions on 500kHz by the carrier at the top of our allocation on 504kHz. Most screen captures and On-line Grabbers show this at varying strengths.
My Kenwood TS870 shows very little of the carrier using a long wire antenna, using my tuned 600m TX antenna on RX there is no sign of the signal.

Finally I have conducted some comparison test with the Kenwood TS870, Yaesu FT897, and Icom IC706. The screen shots show the three rigs using my 40m Doublet antenna with a Ferrite Slab coil in series roughly tuned to 500kHz. The pictures can be viewed full size by, right click on picture -view image

First the FT897. (the wide near vertical lines are from my LCD monitor)
The 504kHz carrier shows red.
Many odd wide horizontal lines are evident.
Reception of in band WSPR signals is good

Next the TS870, same antenna, no carrier on 504kHz, no horizontal crud. (again he wide, near vertical lines are from the LCD monitor) In band WSPR signals are good, maybe a dB or so up on the small rigs.

The IC706 MK nothing same antenna.(the monitor crud is almost horizontal)
The 504kHz carrier is bright green.
There are wide horizontal crud lines, not strong.
in band WSPR signals are good.

The IC706 again to prove a point. First half of the display is with the Doublet, second half is with the tuned 600m TX antenna. In band WSPR signals are stronger using the purpose 600m antenna, the 504kHz carrier is reduced.

My conclusions are that contrary to popular belief there is no radiated harmonic on 504kHz, it is a mixer product generated within the receiver, particularly the small portable toy rigs.

The short wide horizontal noise lines also experienced on these rigs are mixer products too.

As a final test I tuned my doublet to 252kHz and fed the TS870 with the signal.
Looks familiar the rig can be forced into the same poor performance as the toys.

The wide crud lines have a similar frequency distribution as seen in many screen shots.

Short Distance 500kHz Success

After a bit of discussion on 70 cm this morning, we talked through Spectran and WSPR settings and use. I was convinced that any wire antenna in an FT817 would produce some signals on 500kHz. With 6Km between us G3KAN had just got to see more than just a 30 second burst of my QRSS.

At 14.22 we made it, using just the wire in the rig, the signal -15dB.

2010-01-2814:42

G3ZJO

0.503820

-15

0

IO92ng

+17

0.050

G3KAN

IO92

26

A run of 4 Spots followed, then some screen shots by e-mail. It will be interesting to find out how long the RX was on for when we talk again. I am intrigued by the possibility that short periods of enhanced propagation bring signals up to a usable level on a poor antenna at short range.

Activity on 500kHz Increasing

500kHz (600m) Activity is increasing slowly as more stations in more countries get active. Once your Antenna efficiency RX or TX reaches a certain level the circle of possible contacts expands. The number of of stations logged quickly get to a level and new ones are slow to come in.

I am pleased to have a report overnight from the POLICE, well the call always looks like that and makes it memorable, I think, let me try, PA0LCE, yes that's right at 423Km and -33dB on WSPR so right on the edge of de-code-ability.

There are several more bridges to cross. Local stations at 5 to 6 Km and 15 Km who report, no see, although there may have been an odd 30 seconds of good signal at 6Km the other night, not enough QRSS to be sure. These local distances intrigue me.

500kHz QRP Sucessful Station

After much experimentation I have settled on a set up for 500kHz that is giving me some good success.


The Antenna is an Inverted L, 6m Vertical and 8m Horizontal. The more Top Loading wire the better, however I don't have the space for several top wires. I added a lumped capacitance in the form of a disk 270mm dia and wire spokes.

The Loading coil is quite a large diameter wound with Silver Plated PTFE Insulated wire.




The PNP NPN pair PA produces some 500 or 600mW of RF at 50 Ohms. This gives an ERP of 200 to 275uW.



Results mainly using WSPR has brought reports from 32 different stations in 7 Countries.